Paralleling of active filters with independent controls

ABSTRACT

A parallel filter arrangement with at least two filters supplying current in line side sensing configuration and a number of sensors for measuring current. The sensors are used to determine the amount of current being supplied by the filters and the amount of current being supplied by a source. The filters adjust their supplied current in order to reduce or eliminate the amount of reactive or harmonic current being supplied by a source.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional application based on U.S. applicationSer. No. 14/146,324, filed Jan. 2, 2014, and further claims the benefitof U.S. Provisional Patent Application Ser. No. 61/748,382, filed Jan.2, 2013, the disclosures of which are hereby incorporated by referenceherein in their entirety for all purposes.

FIELD OF THE INVENTION

The present invention relates generally to the field of controllingelectrical devices that supply current into the electrical system. Moreparticularly, the present invention relates to compensating for theharmonic or reactive currents drawn by non-linear electric loads withmultiple parallel active filters with independent controls.

BACKGROUND OF THE INVENTION

Industrial plants often include power consuming devices such as, forexample, electric motors, pumps, compressors and/or HVAC systems. Thesedevices are often referred to as loads. Many industrial plants controlthe loads of their facility with electrical power converters to improveprocess control and increase energy efficiency such as, for examplethrough the regulation of variable speed devices and the minimization ofinefficient power consumption. Power converters typically behave asnon-linear loads. A non-linear load draws distorted input current atmultiple frequencies from the electrical power source, whether thatsource is supplied by a utility or a local generator.

As seen in FIG. 1, distorted currents 14 are currents that have at leasta fundamental component 10 and a harmonic component 12. The fundamentalcomponent 10 delivers the energy for the load to do useful work.Although necessary for non-linear loads, the harmonic component 12 ofthe current(s) 14 performs no useful work.

The harmonic component 12 is harmful to utility transformers, localgenerators and other electric loads on the same electric supply as theycause excessive heating, voltage distortion on the electrical supply andpotentially impact operation of other equipment sharing the powersource.

In order or keep the harmonic component drawn from a source at a safelevel, electric utilities and end users are adopting uniform powerquality standards such as IEEE-519. One way electric utilities and endusers are complying with uniform power quality standards is by usingharmonic filters to locally source the harmonic component needed by thenon-linear load. If a harmonic filter supplies the harmonic componentrequired by the non-linear load, the harmonic component supplied by thepower source is significantly reduced or eliminated.

One type of filter being used to comply with uniform power qualitystandards is an electronic active filter. Electronic active filterseffectively act as a local harmonic component source to supply thenecessary harmonic component to non-linear loads. Such electronic activefilters have been typically used as a shunt as shown in FIG. 2. Theelectronic active filter 16 operates as a shunt connected current sourceby creating an output current, I_(AF) 18, for supplying the harmoniccomponent 12 for the non-linear load(s) 20. In this arrangement, theelectronic active filter 16 produces the harmonic current 12 drawn bythe non-linear load(s) 20, eliminating a harmonic component from beingdrawn from the source 22. As a result, the source 22 supplies a sourcecurrent 24, via I_(source), containing the fundamental current 10 inaccordance with the uniform power quality standards.

Shunt electronic active filters generally have two main structures, apower circuit 26 and an independent control system 28, as seen in FIG.3. The power circuit 26 is used to produce the harmonic component 12 andinject the harmonic component into the electrical system. Theindependent control system 28 is used to determine what harmoniccomponent 12 should be produced, referred to as current reference, andcontrol the power circuit to accurately produce and track the currentreference(s). The shunt electronic active filter could also be used toproduce a volt-ampere reactive.

As seen in FIG. 3, the independent control system 28 generally consistsof an outer loop regulator 30, an inner current regulator 32 and avoltage modulator 34. The outer loop regulator 30 receives thecurrent(s) 14 of the electrical system desired to be filtered. Basedupon the current(s) 14, the outer loop regulator 30 generates a filterreference signal for the current, that is, current reference 36. Theinner current regulator 32 receives the current reference 36 as well asfeedback measurements 39 of the output of the electronic active filter16. Based upon the current reference 36 and feedback measurement 39, theinner current regulator 32 generates a voltage reference 38. The voltagemodulator 34 receives the voltage reference 38 and converts that voltagereference 38 to gate signals 40 that are output to the power circuit 26.

FIG. 4 shows the schematic of an exemplar electronic active filter powercircuit 26. The illustrated power circuit 26 is capable of injecting3-phase harmonic currents (e.g. I_(AF) _(—) _(A), I_(AF) _(—) _(B),I_(AF) _(—) _(C)) into a 3-phase electrical system; however other powercircuits are known in the industry and the use of such power circuitswould not depart from the spirit of the invention.

The illustrated power circuit 26 contains a two level DC to AC powerconverter 42 consisting of the DC bus capacitor, C_(DC) 44, and sixpower electronic switches, Q₁₋₆ collectively 46. The switches 46 can beof any type, but are shown for explanatory purpose as IGBTs. The IGBTsshown are controlled by gate signals to turn on and turn off atswitching frequencies higher than the frequency of the electricalsystem's fundamental component 10, as determined by the independentcontrol system 28, to produce voltages Vpole_(—A), Vpole_(—B),Vpole_(—C).

A three-phase low pass LCL filter (e.g. L₁, C₁, L₂) 47 converts each ofthe voltages Vpole_(—A), Vpole_(—B), Vpole_(—c), into the three-phaseoutput currents (e.g. I_(AF) _(—) _(A), I_(AF) _(—) _(B), I_(AF) _(—)_(C)). The filter 47 locally filters out extraneous or unwantedcurrents, such as the high frequency switching ripple current, butallows the lower frequency harmonic currents to pass into the electricalsystem. The control system 28 determines the pattern of IGBT gatesignals (G_(Q1)-G_(Q6)) 40 that most accurately produce the necessaryharmonic component 12 in the active filter output current 18.

The current(s) 14 of the electrical system desired to be filtered can bedetermined and supplied to the outer loop regulator 30 of theindependent control system 28 in a number of different ways. The twomost common ways for a single, e.g. non-paralleled, electronic activefilter to obtain the current(s) 14 of the electrical system desired tobe filtered are load side sensing and line side sensing.

Load side sensing is an open loop control method in which the loadcurrent (I_(Load)) is directly or indirectly sensed. FIG. 5 shows anexample of direct sensing of the load side. The load current(s) 14,I_(Load), is sensed for example, by a current sensor 50. Although acurrent sensor is described, the term is intended in a broad sense, anda number of devices are known in the industry to sense current, e.g. atransducer. The sensed current(s) 48 of the load current(s) 14, broadlydefined as the sensed current itself or at least a signal representingor indicating that current or the level or value of that current or acomponent of that current, is received by the outer loop regulator 30.The outer loop regulator 30 extracts the fundamental component 10 fromthe sensed current(s) 48. The extraction of fundamental component 10 canbe done by a high pass filter although other devices are known in theindustry. The fundamental component 10 can be determined by a number ofmethods known in the industry such as an adaptive notch filter with aphase lock loop to determine the notch frequency.

After the fundamental component 10 is stripped from the sensedcurrent(s) 48, the harmonic component 12 of the sensed current 48 isused to output a current reference 36 to the inner current regulator 32.The filter output current 18, e.g. I_(AF), of the power circuit 26 issensed for example, by a current sensor 52, and provided to the innercurrent regulator 32. Here again, the output of the current sensor 52 isbroadly defined as the sensed current itself, a component thereof or atleast a signal representing or indicating that current or the level orvalue of that current. A summation junction 54 of the inner currentregulator 32 compares the current reference 36 to the sensed currentfeedback 39 to determine a comparison or error 56 which is sent to acompensator 58, G, such as for example via a comparison signal. Theinner current regulator 32 is represented in FIG. 5 as a standard closedloop regulator although other methods for regulating the power circuitare known and used in the industry. The compensator 58 processes theerror 56 and outputs a voltage reference 38. The voltage modulator 34receives the voltage reference 38 and, based on that voltage reference,outputs gate signals 40 to the power circuit 26. Power circuit 26thereby outputs a current 18 to the electrical system as describedabove. From the point where the current reference 36 is output to theinner current regulator 32, to the point where a current 18 is output bythe power circuit 26, is indicated as a dashed box 60, which will bereferred to as the inner electronic active filter 60. The deviceenclosed by dashed box 61 will herein be referred to as the load sideelectronic active filter 61.

The compensator 58 could be designed for example, to meet currenttracking performance metrics. A couple of exemplary or commoncompensator implementations include proportional; proportional andintegral; and proportional, integral and differential compensators.Other implementations are known in the industry and could also be usedwithout departing from the spirit of the invention. The harmoniccomponent demand of the load current(s) 14 is supplied by the electronicactive filter 61, thus eliminating the harmonic components from beingsupplied from the source 22.

Load side sensing can be beneficial because it can be relativelystraight forward to implement in state of the art power convertercontrollers and because multiple active filters can be paralleled usingthis control method to reach higher current levels as described furtherbelow. However, load side sensing is an open loop control method whichhas inherent inaccuracies and is sensitive to open loop errors. Forexample, any errors in the current sensors 50, 52 or in theimplementation of the inner current regulator 32 can lead to currentregulator tracking errors and remnant harmonic currents in the source22. Also, the physical installation of load side sensors can bedifficult in certain applications, such as motor control centers wherethe load electrical bus is not easily accessible, or where multiplenon-linear loads are present.

Line side sensing is an alternate method that overcomes many of theproblems associated with load side sensing. As shown in FIG. 6, lineside sensing is a closed loop control method wherein the sensedcurrent(s) 48 of the source current 24, I_(Source), is sensed forexample, by a current sensor 50. The voltage could also be sensed, forexample, in order to determine the fundamental frequency. Additionalelectrical system quantities could also be sensed with addition sensors.Because line side sensing is a closed loop control method, it is not assensitive to open loop errors as is load side sensing and can yieldbetter performance due to the closed loop control action. Further, lineside sensing is usually easier to install because the AC voltage sourcebus in a facility is often more accessible for installing currentsensors. Line side sensing also provides filtering for all non-linearloads present.

Once the current(s) of the source current 24 is sensed, the sensedcurrent(s) 48 is sent to a filter controller 62. The filter controller62 removes the fundamental component 10 and outputs the harmoniccomponent 12 as a feedback 64 to the outer loop regulator 30.

In addition to receiving the harmonic component feedback 64 of thesource current 24, the outer loop regulator 30 also receives a filterreference 66. Because it is desired in this illustrated example, thatthe source 22 supply no harmonic component 12, the filter reference 66is set to zero. The summation junction 68 of the outer loop regulator 30compares the harmonic component feedback 64 to the filter reference 66to determine a comparison or error 70 which is sent to a compensator 71,G₁, such as for example via a comparison signal. The compensator 71processes the error 70 and outputs a current reference 36. Due to theclosed loop action, the outer loop regulator 30 outputs anoften-adjusted current reference 36 to drive down the harmonic componentfeedback 64 being supplied by the source 22. At steady state, thecurrent reference 36 is equal to the harmonic component 12 drawn by thenon-linear load 20. Once current reference 36 is output, the innerelectronic active filter 60 operates as previously described withreference to FIG. 5. Although the prior art circuit shown in FIG. 5 isshown and described using a filter reference 66, other means forgenerating an error 70 are known and used in the industry, includingusing no harmonic reference at all. From the point at which a feedback64 is supplied to the outer loop regulator 30 up through the point thata current 18 is output by the power circuit 26 will be referred to asthe line side electronic active filter 72.

Generally electronic active filters are rated based on their outputcurrent capacity. The necessary capacity of the electronic activefilter(s) is based on the amount of harmonic component 12 in the loadcurrent(s) 14. In many applications, the amount of harmonic correctioncurrent needed to eliminate harmonic current from the source 22 exceedsthe capacity of a single electronic active filter. In these cases,multiple electronic active filters with independent control systems aredeployed in parallel using a combination of the line side and load sidesensing.

FIG. 7 shows an example of parallel electronic active filters whereinall the electronic active filters are load line sensing. Because loadside sensing is an open loop control method, as referred to above,multiple electronic active filters can be placed in parallel. FIG. 7illustrates an exemplary embodiment wherein two load side electronicactive filters 61, 61′ are shown. A current sensor 50 senses the loadcurrent(s) 14 and outputs the sensed current 48 to both load sideelectronic active filters 61, 61′. Before the sensed current 48 isreceived by the outer current regulators of the load side electronicactive filters 61, 61′, the sensed current 48 is divided by the numberof load side electronic active filters. Therefore, in a system with Nparallel load side electronic active filters, each load side electronicactive filter will operate on 1/N^(th) of the sensed current(s) 48 ofthe load current(s) 14 and supply to the electrical system via itsharmonic component output 18 1/N^(th) of the harmonic component 12 drawnby the non-linear load 20. The example illustrated in FIG. 7 isperformed entirely using an open loop control method and therefore, asdescribed above, has the inherent performance limitations of a singleopen loop active filter control method described above and, in fact,would be compounded based on the use of additional load side electronicactive filters.

Another example of parallel electronic active filters is shown in FIG.8. The example illustrated in FIG. 8 has one line side electronic activefilter 72 and one load side electronic active filter 61. However, anynumber of load side sensing electronic active filters could be addedbecause, as described above, load side sensing is an open loop controlmethod and there is no conflict. In the embodiment shown in FIG. 8, acurrent sensor 50 senses the source current 24 and outputs the sensedcurrent 48 to the fundamental extractor or filter controller 62 of theline side electronic active filter 72. Thereafter, line side electronicactive filter 72 operates as described above. Another current sensor 50′senses the load current(s) 14 and outputs the sensed current 48′ to theload side electronic active filter 61. Thereafter, the load sideelectronic active filter 61 operates as described above. Although theexample illustrated in FIG. 8 is not performed entirely using an openloop control method, it is partially open loop, and to that extent stillhas the inherent performance limitations of a single open loop activefilter described above.

Yet another example of parallel electronic active filters is shown inFIG. 9, in which the load current(s) 14 is synthesized. This arrangementis used when the load bus is inaccessible for load side sensing. In thisembodiment, the current sensor 50 outputs the sensed current 48 of thesource current 24 to a summing junction 76 and also to a fundamentalextractor or filter controller 62 of the line side electronic activefilter 72. Thereafter, line side electronic active filter 72 acts aspreviously described above. A current sensor 75 senses the sum current74 of the currents 18, 18′ being output by the electronic active filters72, 61 respectively. The sum current 74 is output to the summationjunction 76 and is compared to the sensed current(s) 48, the result ofwhich is called the synthesized load current 78. Summing junction 76could be, for example, a current sensor, or the function could beaccomplished by a microprocessor. The synthesized load current 78 issent to the load side electronic active filter 61, which operates asdescribed above. In the example illustrated in FIG. 9, the totalharmonic component or sum current 74 is measured directly with onecurrent sensor 75; however, the sum current 74 could be determined byusing a separate current sensor, e.g. 52, 52′ to sense each output 18,18′ and sum the harmonic components such as, for example, by a summingjunction. Although the example illustrated in FIG. 9 has a line sideelectronic active filter 72, it still has the inherent performancelimitations of a single open loop due to the load side electronic activefilter 61 being, as described above, set up in an open loopconfiguration.

Paralleling line side electronic active filters is not currently known,because any arrangement now known would result in uncontrolled andunacceptable circulating currents between filters, thereby reducingperformance. A circulating current between electronic active filters iscurrent that flows between filters and but does not cancel the loadharmonic component being drawn from the source. Because each electronicactive filter has a maximum current it is capable of producing, theadditional circulating current reduces the current available to supplythe harmonic component being drawn by the non-linear load, therebyallowing the harmonic component to be drawn from the source. As is seenfrom the examples provided herein, therefore, currently all parallelingschemes for multiple electronic active filters require some or all ofthe electronic active filters be configured in a load side sensingarrangement, which, as described further above, has inherent performancedrawbacks.

As a result, there exists a need to parallel all electronic activefilters in a line side sensing arrangement to capture the performancebenefits of the closed loop control method described above, while stillavoiding unacceptable circulating currents.

It will be understood by those skilled in the art that one or moreaspects of this invention can meet certain objectives, while one or moreother aspects can lead to certain other objectives. Other objects,features, benefits and advantages of the present invention will beapparent in this summary and descriptions of the disclosed embodiment,and will be readily apparent to those skilled in the art. Such objects,features, benefits and advantages will be apparent from the above astaken in conjunction with the accompanying figures and all reasonableinferences to be drawn therefrom.

SUMMARY OF THE INVENTION

The invention provides a parallel filter circuit for use with anelectrical system having a number of filters and sensors, the electricalsystem being capable of connecting to a power source and capable ofhaving at least one load connected thereto. The first filter is capableof producing a first current and connected to the electrical system at afirst location downstream of the first current sensor. The second filteris capable of producing a second current at a second location downstreamof the first current sensor. The first current sensor is capable ofsensing at least a current of the electrical system, produces a firstsignal indicating the current of the electrical system and sends thefirst signal to the first and second filters. The second current sensoris capable of sensing the first current, produces a second signalindicating the first current and sends the second signal to the firstfilter. The third current sensor is capable of sensing the secondcurrent, produces a third signal indicating the second current and sendsthe third signal to the second filter. The first filter produces thefirst current and supplies the first current to the electrical systemthrough the first location based at least in part upon the first signal,the second signal and a difference between the first current and thesecond current. The second filter produces the second current andsupplies the second current to the electric system through the secondlocation based at least in part upon the first signal, the third signaland the difference between the first current and the second current.

The present invention also relates to a method of reducing circulatingcurrent between two line side sensing electronic active filters in anelectrical system that has a current source supplying current to a load.A current from the source is sensed. A first and a second currentcomponent are generated and sensed, and the difference between them isdetermined. The difference and the sensed current from the source arecompared with a filter reference signal to arrive at a comparison. Thefirst current component is adjusted using the comparison and the sensedfirst current component. The second current component is adjusted usingthe comparison and the sensed second current component.

In another embodiment the parallel filter arrangement is used in acircuit with at least one load. The parallel filter arrangement includesa first filter capable of producing a first current, a second filtercapable of producing a second current and a first, second and thirdcurrent sensor. The first current sensor produces a first signalindicating the first current. The second current sensor produces asecond signal indicating the second current. The third sensor produced athird signal representing a current of the circuit located upstream fromthe first and second filters. The first filter produces the firstcurrent and supplies the first current to the circuit based at least inpart upon the first signal and third signal. The second filter producesthe second current and supplies the second current to the circuit basedat least in part upon the second signal and the difference between thefirst signal and the third signal.

The invention also relates to a method of reducing circulating currentbetween two line side sensing electronic active filters in an electricalsystem that has a current source supplying current to a load. A firstcurrent from the source is sensed at a first location. A second currentfrom the source is sensed at a second location downstream from the firstlocation. A first current component is generated, supplied downstreamfrom the first location and sensed. A second current component isgenerated, supplied downstream from the second location and sensed. Thefirst current component is adjusted using the sensed first current andthe sensed first current component. The second current component isadjusted using the sensed second current and the sensed second currentcomponent.

In yet another embodiment, the parallel filter arrangement is used in acircuit with at least one load, with the parallel filter arrangementincluding a first filter capable of producing a first current, a secondfilter capable of producing a second current and a first, second, thirdand fourth current sensor. The first current sensor produces a firstsignal indicating the first current. The second current sensor producesa second signal indicating the second current. The third current sensorproduces a third signal representing a current of the circuit locatedupstream from the first and second filters. The fourth current sensorproduces a fourth signal representing a current of the circuit locatedupstream from the second filter and downstream from the first filter.The first filter produces the first current and supplies the firstcurrent to the circuit based at least in part upon the first signal andthe third signal. The second filter produces the second current andsupplies the second current to the circuit based at least in part uponthe second signal and the fourth signal.

The invention further relates to a method of reducing circulatingcurrent between two line side sensing electronic active filters, whereina current from the source is sensed. A first and second currentcomponent are generated and sensed. The first current component isadjusted using the sensed current from the source, the sensed firstcurrent component and a filter reference. The second current componentis adjusted using the sensed current from the source, the sensed secondcurrent component, the sensed first current component and a filterreference.

The present invention also relates to a method of filtering a currentdrawn by a load from a current source that is providing a current in anelectrical system, the electrical system having a first line sidesensing electronic active filter and a second line side sensingelectronic active filter. A current from the source is sensed. Afundamental current component and a harmonic current component aregenerated from the current sensed from the current source. At least aportion of the harmonic current component from the first line sidesensing electronic active filter is supplied by means of supplying afirst current component, and at least a portion of the harmonic currentcomponent from the second line side sensing electronic active filter issupplied by means of supplying a second current component. A differencebetween the first current component and the second current component isdetermined. The difference, the current from the current source and afilter reference signal is compared to arrive at a comparison signal.The first current component is adjusted using the comparison signal andthe sensed first current component. The second current component isadjusted using the comparison signal and the sensed second currentcomponent.

Other objects and advantages of the invention will become apparenthereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a series of conventional and fundamental charts showing someof the individual components comprising input current as a function oftime.

FIG. 2 is a schematic diagram of a known shunt electronic active filterarrangement.

FIG. 3 is a schematic diagram of a conventional independent controlsystem of an electronic active filter.

FIG. 4 is a schematic diagram of a known power circuit of an electronicactive filter.

FIG. 5 is a schematic diagram of a prior art electronic active filter ina load side sensing arrangement.

FIG. 6 is a schematic diagram of a prior art electronic active filter ina line side sensing arrangement.

FIG. 7 is a schematic diagram of paralleling electronic active filtersin a prior art load side sensing arrangement.

FIG. 8 is a schematic diagram of a prior art paralleling of electronicactive filters in a load side sensing and line side sensing combinationarrangement.

FIG. 9 is a schematic diagram of a prior art paralleling of electronicactive filters in line side sensing and synthesized load line sensingarrangement.

FIG. 10 is a schematic diagram of paralleling two electronic activefilters in line side sensing arrangement with current differencefeedback according to one embodiment of the present invention.

FIG. 11 is a schematic diagram of paralleling four electronic activefilters in line side sensing arrangement with current differencefeedback according to another embodiment of the invention.

FIG. 12 is a schematic diagram of paralleling electronic active filtersin line side sensing with cascading source sensing arrangement inaccordance with another embodiment of the present invention.

FIG. 13 is a schematic diagram of paralleling electronic active filtersin line sensing arrangement with cascading source sensing and multipleload locations, in accord with still another embodiment of the presentinvention.

FIG. 14 is a schematic diagram of paralleling electronic active filtersin line sensing arrangement with synthesized cascading source sensing,according to yet another embodiment of the present invention.

DETAILED DESCRIPTION

Reference numerals appearing below that are the same as referencenumerals appearing above refer to the same elements, including circuitelements, currents, and so on.

Referring now to FIG. 10, a parallel electronic active filter circuit 79constructed according to one embodiment of the present invention hasmultiple electronic active filters configured in a line side sensingconfiguration. As shown in FIG. 10, a current sensor 50 senses thecurrent of the source current 24 and outputs the sensed current 48, suchas for example via a signal, to the first line side electronic activefilter 72, and also to the second line side electronic active filter72′. Another current sensor 75 is used to sense the difference or error80 between the current 18 output by the first line side electronicactive filter 72 and the current 18′ output by the second line sideelectronic active filter 72′. In the illustrated example, one of theoutputs is looped through sensor 75 so as to be inverted with respect tothe other output, and thereby create a differential or error 80. In theexample circuit shown in FIG. 10, it is the harmonic current output 18′that is so looped as to be inverse to the harmonic current output 18. Asjust described, the current sensor 75 outputs this error 80, such as forexample via a signal, to the first line side electronic active filter72, and also to the second line side electronic active filter 72′. Theoutput of the current sensor 75 is broadly defined as the differencebetween the sensed currents itself, a component thereof or at least asignal representing or indicating that current or the level or value ofthat current.

The sensed current 48, derived as described earlier herein, is receivedby the first outer loop regulator 30. In the example shown in FIG. 10,the fundamental extractor or filter controller 62 previously describedwith respect to, for example, the single line side sensing electronicactive filter shown in FIG. 6, is included in the outer loop regulators30, 30′. At the summing junction 68 of the outer loop regulator 30, thesensed current(s) 48 is compared against the combination of the error 80and a filter reference 66. As previously described, the filter reference66 is set to zero in the circuit shown in FIG. 10 because it is desiredthat the source 22 supply no harmonic component 12. The summing junction68 outputs the comparison or error 70 to a compensator 71, such as forexample via a comparison signal. The compensator 71 generates a currentreference 36 and outputs it to the inner current regulator 32. Oncecurrent reference 36 is output, the inner electronic active filter 60 isas previously described.

At the summing junction 68′ of the outer loop regulator 30′ the filterreference 66′ is compared to the combination of the harmonic componentof the sensed current(s) 48 and the error 80. The summing junction 68′outputs the comparison or error 70′ to a compensator 71′, such as forexample via a comparison signal. The compensator 71′ generates a currentreference 36′ and outputs it to the inner current regulator 32′. Oncethe current reference 36′ is output, the inner electronic active filter60′ is as previously described.

The embodiment described above with regards to FIG. 10, provides controlstructures that allow multiple electronic active filters to beparalleled on an electrical power system and controlled using theindependent and closed loop, line side sensing arrangement. To avoidcirculating currents present in current parallel electronic activefilter arrangements, this invention integrates the electronic activefilter's internal harmonic current regulator functionality with acontrol function to regulate the current difference 80 betweenelectronic active filters to a minimum. For the two parallel electronicactive filter embodiment shown in FIG. 10, the circulating current canbe defined as the difference, or error 80, between the respectiveharmonic current outputs 18, 18′ of the two parallel electronic activefilters. It then follows that if the harmonic current 18, 18′ of eachelectronic active filter respectively, is controlled to be the same,e.g. harmonic component output 18 equals harmonic current output 18′,then the error 80 is zero, as is the circulating current.

As described, the electronic active filters 72, 72′ of the embodimentdescribed above use the error 80 together with the sensed current(s) 48to adjust their respective current outputs 18, 18′. For example, in asteady state, let the total harmonic component 12 demanded from thenon-linear load 20 be I_(H). In steady state, the first and secondelectronic active filters together provide the harmonic component 12demanded, via harmonic current outputs 18, 18′, eliminating the harmoniccomponent 12 drawn from the source 22. If the first electronic activefilter delivers more current than the second electronic active filter,the error 80 is positive. The positive error together with the sensedcurrent(s) 48 will effectively lower the harmonic current reference 36to the first electronic active filter causing the first electronicactive filter to adjust its harmonic current output 18 lower.Simultaneously, the positive error together with the sensed current(s)48 will increase the harmonic current reference 36′ to the secondelectronic active filter causing the second electronic active filter toadjust its harmonic current output 18′ higher. Thus, this mechanism willcause the two electronic active filters to balance their harmoniccurrent outputs 18, 18′ thereby eliminating or significantly reducingany circulating current therebetween. In the embodiment described abovewith regards to FIG. 10, the electronic active filters 72, 72′ canadditionally use the feedback measurements 39, 39′ of their outputs18,18′ and/or a filter reference 66, 66′ to adjust their respectivecurrent outputs 18, 18′.

The embodiment described with regards to FIG. 10 could be extended toparalleling a number of electronic active filters to the power of two inline side sensing (e.g. 2^(N) electronic active filters where N is aninteger and 2^(N)=2, 4, 8, 16 . . . etc). In extending the number ofline side electronic active filters, the number of summing junctions tocalculate current difference error between filters is 2^(N)−1 and thenumber of error inputs to each line side electronic active filters is N.One example of the extension to paralleling a number of electronicactive filters is shown in FIG. 11 wherein four line side electronicactive filters 72, 72′, 72″, 72′″ are described.

As with the embodiment described in FIG. 10 involving two electronicactive filters, the embodiment described with respect to FIG. 11 has acurrent sensor 50 that senses the current of the source current 24 andoutputs the sensed current 48, such as for example via a signal, torespective summing junctions 82, 82′, 82″, 82′″ of each respective lineside electronic active filter 72, 72′, 72″, 72′″. As described withrespect to FIG. 10, each summing junction 82, 82′, 82″, 82′″ of FIG. 11has a fundamental extractor or filter controller 62 included so as toremove the fundamental component of the sensed current 48.

Each line side electronic active filter 72, 72′, 72″, 72′″ outputs acurrent 18, 18′, 18″, 18′″ respectively. Each current output 18, 18′,18″, 18′″ is sensed by a separate current sensor 84, 84′, 84″, 84′″. Thesensed outputs 18, 18′ of the first and second line side electronicactive filters 72, 72′ are output to a summing junction 86. Thedifference or error 88 between the two harmonic current outputs 18, 18′is determined and sent to summing junctions 82, 82′ of each of the firstand second line side electronic active filters 72, 72′. The output ofthe summing junction 86 is broadly defined as the difference or error 88between the two harmonic current outputs 18, 18′ itself, a componentthereof or at least a signal representing or indicating that current orthe level or value of that current.

The sensed current outputs 18″, 18′″ of the third and fourth line sideelectronic active filters 72″, 72′″ are output to a summing junction86′. The difference or error 88′ between the two current outputs 18″,18′″ is determined and output to the summing junctions 82″, 82′″ of eachof the third and fourth line side electronic active filters 72″, 72″,respectively. The output of the summing junction 86′ is broadly definedas the difference or error 88′ between the two harmonic current outputs18″, 18′″ itself, a component thereof or at least a signal representingor indicating that current or the level or value of that current.

A combination current 90 consisting of the current outputs 18, 18′ ofthe first and second line side electronic active filters 72, 72′ issensed by a current sensor 92. The current sensor 92 outputs the sensedcombination current 94 to a summing junction 96.

A combination harmonic component 90′ consisting of the harmonic currentoutputs 18″, 18′″ of the third and fourth line side electronic activefilters 72″, 72′″ is sensed by a current sensor 92′. The current sensor92′ outputs the sensed combination harmonic component 94′ to the summingjunction 96. The summing junction 96 determines the difference or error98 between the sensed combination harmonic component 94 and the sensedcombination harmonic component 94′ and outputs the error 98 to thesumming junctions 82, 82′, 82″, 82′″ of each line side electronic activefilter 72, 72′, 72″, 72′″ respectively. The output of the summingjunction 96 is broadly defined as the difference or error 98 between thetwo combination harmonic components 90, 90′ itself, a component thereofor at least a signal representing or indicating that current or thelevel or value of that current.

The summing junction 82 of the first line side electronic active filter72 determines the difference or error 100 between the error 98 and thecombination of the sensed current 48 and the error 88. The error 100 isthen supplied to the outer loop regulator 30 of the first line sideelectronic active filter 72.

The summing junction 82′ of the second line side electronic activefilter 72′ determines the difference or error 100′ between the error 98and the combination of the sensed current 48 and the error 88. The error100′ is then supplied to the outer loop regulator 30′ of the second lineside electronic active filter 72′.

The summing junction 82″ of the third line side electronic active filter72″ determines the difference or error 100″ between the error 88′ andthe combination of the sensed current 48 and the error 98. The error100″ is then supplied to the outer loop regulator 30″ of the third lineside electronic active filter 72″.

The summing junction 82″ of the fourth line side electronic activefilter 72″ determines the sum or error 100′″ of the sensed current 48,the error 88′ and the error 98. The error 100′″ is then supplied to theouter loop regulator 30′″ of the fourth line side electronic activefilter 72′.

Once the respective errors 100, 100′,100″, 100′ are output to therespective outer loop regulators 30, 30′, 30″, 30′″ of each respectiveline side electronic active filter 72, 72′, 72″, 72′, the line sideelectronic active filters 72, 72′, 72″, 72″ operate as previouslydescribed with regards to line side electronic active filter 72 in FIG.6 wherein the respective errors 100, 100′,100″, 100′″ are compared to afilter reference.

As described above, each pair of electronic active filters, e.g. thefirst and second line side electronic active filters 72, 72′, receivesthe outputs of the sensed current 48 and the error difference betweenthose two electronic active filters, in this example, error 88, as theywould if only the two line side electronic active filters 72, 72′ werein parallel. In addition, to control the circulating current betweeneach pair of electronic active filters, the error 98 between each pairof electronic active filters is received by each electronic activefilter. This could be applied to virtually any number of electronicactive filters to the power of two in a line side sensing configuration.

Although the exemplary embodiment shown and described in FIG. 11 showsone way of measuring, calculating and receiving the error, e.g. 88,other ways are possible. For example the error, in this example error88, could be measured using one current transducer for the currentoutput, e.g. 18, 18′, of each pair of line side electronic activefilters, e.g. 72, 72′ such as described with respect to the currentsensor 75 shown in FIG. 10. Further, the error, e.g. 88, and the sensedcurrent 48 could each be sent directly to the outer loop regulator 30,30′, of each line side electronic active filter 72, 72′ respectively.Another alternative would be to send the sensed current output, e.g. 18,18′, 18″, 18′″ from each of the current sensors, e.g. 84, 84′, 84″,84′″, to a single microcontroller that can be programmed to compute allthe desired errors and output them to the desired electronic activefilter. Furthermore, electronic active filters of different currentratings could be paralleled using a feedback scaling factor applied toerror, e.g. 88, prior to being output to the outer loop regulator 30,30′ of each line side electronic active filter 72, 72′, respectively.

The paralleling of electronic active filters can also be accomplishedusing a cascading line side sensing configuration. Referring to FIG. 12,a current sensor 50 senses the source current 24 from which one or morenon-linear loads 20′ are drawing current, and outputs the sensed current48, such as for example via a signal, to the outer loop regulator 30 ofthe first line side electronic active filter 72. Once sensed current 48is output to the outer loop regulator 30 of the line side electronicactive filter 72, the line side electronic active filter 72 operates aspreviously described, supplying the harmonic current output 18 atlocation 106, downstream from current sensor 50.

A second current sensor 50′ senses the current, I_(Source2), of thesource current 24 at a location 102 downstream from location 106, andupstream from where the harmonic current output 18′ of the second lineside electronic active filter 72′ is supplied to the electrical systemat location 104. The second sensed current 48′ is output, such as forexample via a signal by second current sensor 50′, to the outer loopregulator 30′ of the second line side electronic active filter 72′. Oncethe second sensed current 48′ is output to the outer loop regulator 30′of the line side electronic active filter 72′, the line side electronicactive filter 72′ operates as previously described with regards to theline side electronic active filter 72.

Because the current sensor 50′ is downstream of both the current sensor50 and the location 106 where the current output 18 of the first lineside electronic active filter 72 is supplied to the electrical system,the sensed current 48 is equal to the difference of the sensed current48′ and the harmonic current output 18 of the first line side electronicactive filter 72.

The cascaded line side sensing arrangement is effective in minimizingcirculating current between electronic active filters because of theseparate locations of the current sensors 50, 50′. The second line sideelectronic active filter 72′ supplies its harmonic current output 18′ tocancel any harmonic component 12 drawn by the set of non-linear loads20′, independent of and unaffected by the harmonic current output 18 ofthe first line side electronic active filter 72. If a portion of theharmonic current output 18′ of the second line side electronic activefilter 72′ did circulate into the first line side electronic activefilter 72, it would be detected by the current sensor 50′ and minimizedby the outer loop regulator 30′ of the second line side electronicactive filter 72′. The first line side electronic active filter 72supplies its harmonic current output 18 to cancel harmonic componentremaining in the sensed current 48′ after the second line sideelectronic active filter 72′ has supplied its harmonic current output18′. The arrangement described with regards to FIG. 12 can be extendedto any practical number of additional line side electronic activefilters upstream of the first line side electronic active filter 72 inthe same manner.

In the embodiment described above with regards to FIG. 11, theelectronic active filters 72, 72′ use the sensed currents 48, 48′together with the feedback measurements 39, 39′ of their outputs 18,18′to adjust their respective current outputs 18, 18′. In this embodiment,the electronic active filters 72, 72′ can additionally use a filterreference 66, 66′ to adjust their respective current outputs 18, 18′.Because of the separate locations of the current sensors 50, 50′, thisarrangement allows for the flexible location of linear and non-linearloads. Both linear and non-linear loads can be placed not only at theend of the electrical system and downstream of all electronic activefilters, e.g. 20′ as shown in FIG. 12, but also interspersed in theelectrical system as shown in FIG. 13.

Referring to FIG. 13, the first and second line side electronic activefilters 72, 72′ are substantially as described with regards to FIG. 12.The set of non-linear loads 20′ shown in FIG. 12 are, in FIG. 13,dispersed throughout the electrical system. In this exemplaryarrangement, all non-linear loads are filtered by one or more electronicactive filters because of the location of the current sensors. In theexemplary embodiment shown in FIG. 13, the harmonic components 12 drawnby the non-linear loads 20 a and 20 b are supplied by the harmoniccurrent output 18′ of the second line side electronic active filter 72′because the non-linear loads 20 a and 20 b are downstream of the currentsensor 50′.

The harmonic components 12 drawn by the non-linear loads 20 c and 20 dare entirely supplied by the harmonic current output 18 of the firstelectronic active filter 72 as are any remnant harmonic component drawnby non-linear loads 20 a and 20 b that are not fully supplied by theharmonic current output 18′ of the second line side electronic activefilter 72. Again, this arrangement works because the non-linear loads 20a-20 d are downstream of the current sensor 50.

The paralleling of electronic active filters can also be accomplishedusing a synthesized cascading line side sensing arrangement.Synthesizing the source current can reduce the size and cost of currentsensors. The current sensor 110 in FIG. 14 for example, will typicallybe smaller and lower cost than the source current sensor 50′ in FIG. 13,because the current 18 is typically a lower amperage than I_(Source2).

Referring to FIG. 14, a current sensor 50 senses the current of thesource current 24 from which a set of non-linear loads 20′ are drawing,and outputs the sensed current 48. In this embodiment, the sensedcurrent 48 is output both to the outer loop regulator 30 of the firstline side electronic active filter 72 and also to a summing junction108. Once sensed current 48 is output to the outer loop regulator 30 ofthe first line side electronic active filter 72, the line sideelectronic active filter 72 operates as previously described. As such,the first electronic active filter 72 uses the sensed current(s) 48together with the feedback measurements 39 of output 18 and a filterreference 66 to adjust its current output 18.

A second current sensor 110 is employed to sense the current output 18being supplied by the first line side electronic active filter 72. Thesecond current sensor 110 outputs the sensed harmonic current 112 to thesumming junction 108. The summing junction 108 sums the sensed harmoniccurrent 112 and the sensed current 48 to create a synthesized sourcecurrent 114. The synthesized source current 114 is fed to the outer loopregulator 30′ of the second line side electronic active filter 72′,which from thereon operates as previously described. As such, the secondelectronic active filter 72′ uses the sensed current(s) 48 together withthe feedback measurements 39′ of output 18′, the sensed harmonic current112 from output 18 and a filter reference 66′ to adjust its currentoutput 18′.

Although the invention has been herein described in what is perceived tobe the most practical and preferred embodiments, it is to be understoodthat the invention is not intended to be limited to the specificembodiments set forth above. For example, many of the illustratedexamples described above relate to the production of a harmonic current.However, the electronic after filter arrangements discussed above couldalso be used to produce other current without departing from the spiritof the invention, such as, for example, volt-ampere reaction. Rather, itis recognized that modifications may be made by one of skill in the artof the invention without departing from the spirit or intent of theinvention and, therefore, the invention is to be taken as including allreasonable equivalents to the subject matter of the appended claims andthe description of the invention herein.

What is claimed is:
 1. A parallel filter arrangement for use with acircuit having at least one load connected thereto comprising: a firstfilter capable of producing a first current; a second filter capable ofproducing a second current; a first current sensor producing a firstsignal indicating the first current; a second current sensor producing asecond signal indicating the second current; a third current sensorproducing a third signal representing a current of the circuit andlocated upstream from the first and second filters; wherein the firstfilter produces the first current and supplies the first current to thecircuit based at least in part upon the first signal and third signal,and the second filter produces the second current and supplies thesecond current to the circuit based at least in part upon the secondsignal and the combination of the first signal with the third signal. 2.The parallel filter circuit of claim 1, wherein the first filterproduces the first current and the second filter produces the secondcurrent based additionally upon a filter reference signal.
 3. Theparallel filter circuit of claim 1, wherein a summing junctiondetermines an error between the first signal and the third signal andsends a signal representing the error to the second filter.
 4. Theparallel filter circuit of claim 1, further comprising a means forsending a signal to the second filter representing an error between thefirst signal and the third signal.
 5. A parallel filter arrangement foruse with a circuit having at least one load connected theretocomprising: a first filter capable of producing a first current; asecond filter capable of producing a second current; a first currentsensor producing a first signal indicating the first current; a secondcurrent sensor producing a second signal indicating the second current;a third current sensor producing a third signal representing a currentof the circuit and located upstream from the first and second filters; afourth current sensor producing a fourth signal representing a currentof the circuit and located upstream from the second filter anddownstream from the first filter; wherein the first filter produces thefirst current and supplies the first current to the circuit based atleast in part upon the first signal and the third signal, and the secondfilter produces the second current and supplies the second current tothe circuit based at least in part upon the second signal and the fourthsignal.
 6. The parallel filter circuit of claim 5, wherein the at leastone load comprises at least two loads located downstream from the thirdcurrent sensor.
 7. The parallel filter circuit of claim 5, wherein thefirst filter produces the first current and the second filter producesthe second current based additionally on a filter reference signal. 8.The parallel filter circuit of claim 5, wherein the first, second, thirdand fourth current sensors are current transducers.
 9. The parallelfilter circuit of claim 5, wherein the first current and the secondcurrent are a current selected from the group of harmonic current andvolt-ampere reactive.